Cell division plays an important role during all phases of plant development. The continuation of organogenesis and growth responses to a changing environment requires precise spatial, temporal and developmental regulation of cell division activity in meristems (and in cells with the capability to form new meristems such as in lateral root formation). Such control of cell division is also important in organs themselves (i.e. separate from meristems per se), for example, in leaf expansion, secondary growth, and endoreduplication.
A complex network controls cell proliferation in eukaryotes. Regulatory pathways communicate environmental constraints, such as nutrient availability, mitogenic signals such as growth factors or hormones, or developmental cues such as the transition from vegetative to reproductive. Ultimately, these regulatory pathways control the timing, frequency (rate), plane and position of cell divisions.
The basic mechanism of cell cycle control is conserved among eukaryotes. A catalytic protein serine/threonine kinase and an activating cyclin subunit control progress through the cell cycle. The protein kinase is generally referred to as a cyclin-dependent-kinase (CDK), whose activity is modulated by phosphorylation and dephosphorylation events and by their association with regulatory subunits, called cyclins. CDKs require association with cyclins for activation, and the timing of activation is largely dependent upon cyclin expression. CDKs are a family of serine/threonine protein kinases that regulate individual cell cycle transitions.
Eukaryote genomes typically encode multiple cyclin and CDK genes. In higher eukaryotes, different members of the CDK family act in different stages of the cell cycle. Cyclin genes are classified according to sequence, the timing of their appearance or activity during the cell cycle, and the cell cycle regulatory proteins with which they interact. In addition to cyclin and CDK subunits, CDKs are often physically associated with other proteins that alter localization, substrate specificity, or activity. A few examples of such CDK interacting proteins are the CDK inhibitors, members of the Retinoblastoma-associated protein (Rb) family, and the Constitutive Kinase Subunit (CKS).
The protein kinase activity of the complex is regulated by feedback control at certain checkpoints. At such checkpoints the CDK activity becomes limiting for further progress. When the feedback control network senses the completion of a checkpoint, CDK is activated and the cell passes through to the next checkpoint. Changes in CDK activity are regulated at multiple levels, including reversible phosphorylation of the cell cycle factors, changes in subcellular localization of the complex, and the rates of synthesis and destruction of limiting components. P. W. Doerner, Cell Cycle Regulation in Plants, Plant Physoil., 106:823-827 (1994).
Plants have unique developmental features that distinguish them from other eukaryotes. Plant cells do not migrate, and thus only cell division, expansion and programmed cell death determine morphogenesis. Organs are formed throughout the entire life span of the plant from specialized regions called meristems. In addition, many differentiated cells have the potential to both dedifferentiate and to reenter the cell cycle. There are also numerous examples of plant cell types that undergo endoreduplication, a process involving nuclear multiplication without cytokinesis. The study of plant cell cycle control genes is expected to contribute to the understanding of these unique phenomena. O. Shaul et al., Regulation of Cell Division in Arabidopsis, Critical Reviews in Plant Sciences, 15(2):97-112 (1996).
Cell division in higher eukaryotes is controlled by two main checkpoints in the cell cycle that prevent the cell from entering either M- or S-phase of the cycle prematurely. Evidence from yeast and mammalian systems has shown that over-expression of key cell cycle activating genes can either trigger cell division in non-dividing cells, or stimulate division in previously dividing cells (i.e. the duration of the cell cycle is decreased and cell size is reduced). Examples of genes whose over-expression has been shown to stimulate cell division include cyclins (see, e.g. Doerner et al., Nature (1996) 380:520-423; Gudas et al., Mol. Cell. Biol. (1999) 19:612-622; Wang et al., Nature (1994) 369:669-671; Quelle et al., Genes Dev. (1993) 7:1559-1571, E2F transcription factors (see, e.g. Johnson et al., Nature (1993) 365:349-352; Lukas et al., (1996) Mol. Cell. Biol. 16:1047-1057), cdc25 (see, e.g. Bell et al., (1993) Plant Molecular Biology 23:445-451; Draetta et al., (1996) BBA 1332:53-63), and mdm2 (see, e.g. Teoh et al., (1997) Blood 90:1982-1992). Conversely, other gene products have been found to participate in negative regulation and/or checkpoint control, effectively blocking or retarding progression through the cell cycle. (see MacLachlan et al., (1995) Critical Rev. Eukaroytic Gene Expression 5(2):127-156).
Current methods for genetic engineering in agronomically important crops such as maize and soybean require a specific cell type as the recipient of new DNA. In maize, these cells are found in relatively undifferentiated, rapidly growing callus cells or on the scutellar surface of the immature embryo (which gives rise to callus). Irrespective of the delivery method currently used, DNA is introduced into literally thousands of cells, yet transformants are recovered at frequencies of 10−5 relative to transiently-expressing cells. In soybean, these cells are found in relatively undifferentiated, rapidly growing callus or suspension cells, or in nodal meristematic regions of the plant. Exacerbating this problem, the trauma that accompanies DNA introduction directs recipient cells into cell cycle arrest and accumulating evidence suggests that many of these cells are directed into apoptosis or programmed cell death. (Bowen et al., Tucson International Mol. Biol. Meetings). It would therefore be desirable to increase transformation efficiency.
Over the period between 1950 and 1980, the increase in maize production worldwide outpaced both wheat and rice. Despite a temporary downswing in the early to mid-1980's (due to both environmental and political factors) world maize production has risen steadily from around 145 million tons in 1950 to nearly 500 million tons by 1990. Increases in yield and harvested area have been the predominant contributors to enhanced world production; with yield playing the major role in industrialized countries and area expansion being most important in developing countries. Yet, over the next ten years it's also predicted that meeting the demand for corn worldwide will require an additional 20% over current production (Dowswell, C. R., Paliwal, R. L., Cantrell, R. P., 1996, Maize in the Third World, Westview Press, Boulder, Colo.).
The components most often associated with maize productivity are grain yield or whole-plant harvest for animal feed (in the forms of silage, fodder, or stover). Thus the relative growth of the vegetative or reproductive organs might be preferred, depending on the ultimate use of the crop. Whether the whole plant or the ear are harvested, overall yield will depend strongly on vigor and growth rate. In modern maize hybrids, the impact of heterosis on overall plant vigor and yield has been unarguably demonstrated (Duvick, D. N.,1984, In: Genetic contributions to yield gains in five major crop plants. W. R. Fehr, ed. CSSA, Madison, Wis.).
Corn breeders since the 1930's have been selectively breeding by identifying inbreds that in combination produce hybrid vigor well beyond either parent. Surprisingly little is known about why hybrids are so much larger than their parent inbreds, although there are some interesting observations in the literature. In metabolic studies, heterosis (increases over either parent) has been observed for physiological traits such as P uptake by roots (Baliger and Barber, 1979; Nielsen and Barber, 1978), but for many enzymatic traits the hybrid is often intermediate to the inbred parents (Hageman, R. H., Leng, E. R., Dudley, J. W. 1967. Adv. Agron. 19:45-86; Chevalier, P., Schrader, L. E. 1977. Crop Sci. 17:897-901; Schrader, L. E. 1974. Crop Sci. 14:201-205; Schrader, L. E. 1985. PP 79-89. In: Exploitation of physiological and genetic variability to enhance crop productivity. Harper, J. E. ed. Am. Soc. Plant Physiol. Rockville, Md., Schrader, L. E., Cataldo, D. A., Peterson, D. M., Vogelzang, R. D. 1974. Plant Physiol. 32:337-341).
Anatomical data is less confusing. In summarizing data from an earlier publication, Kiesselbach states that approximately 10% of the increased vigor of the hybrid over its inbred parents is due to cell enlargement, and 90% can be accounted for simply by increased cell numbers (Kiesselbach, T. A. 1922, 1949. The Structure and Reproduction of Corn, Nebraska Agric. Exp. Stn. Res. Bull. 161). Recently it was shown that overexpressing a B cyclin in Arabidopsis resulted in increased root biomass and the root cells were smaller (indicative of accelerated cell division), but the overall plant morphology was not perturbed (Doerner et al., 1996).